Here you will find information on the crustaceans that can be seen in Lake Geneva.
You can find out more about the crustaceans of Lake Geneva in the following issues 60, 113 of our quarterly magazine Lake Geneva region as well as in our infographics or in our weekly column News in 60 seconds.
Crustaceans are usually associated with the marine environment. However, some species live in Lake Geneva and our rivers, sometimes in very large numbers.
Their bodies are made up of several parts: the cephalonthe head carrying the eyes, antennae and appendages for eating, the thorax and the abdomeneach bearing a pair of appendages or legs and the telsontheir tails.
Crustaceans have gills that allow them to breathe underwater.
The crustaceans of Lake Geneva can be divided into four main genetic orders: Decapods (including crayfish), Amphipods (including Gammares and Corophiidae), Isopods (including Aselles) and Mycidaceans (including the Bloody Red Shrimp).
Decapods, including crayfish, have large pincers that enable them to feed and protect themselves from predators. They generally move around by walking on the bottom, but can also use their claws and tail to propel themselves through the water when in danger.
They reproduce sexually. The eggs are guarded by the female, who collects them on her legs and attaches them to them until they hatch.
Crayfish are omnivores, feeding on small crustaceans, molluscs, plants and even corpses. They roam the lake beds, looking for easy prey. They may also stalk in rocky cavities. They themselves are the prey of certain fish and aquatic birds.
Around 1870, a fungus (Aphanomyces astaci), which lives in the hypodermis and central nervous system, has had a major impact on native crayfish populations in Europe, particularly in Lake Geneva. The American crayfish, Orconectes limosusThe crayfish was then introduced into several areas to replace the native species that had disappeared. It was a bad idea, however, as the healthy carriers of this fungus, also known as the crayfish plague, quickly colonised the different environments and continued to spread the disease.
Amphipods are unique in that they moult throughout their lives.
Gammarids live for 1 to 2 years and can reproduce up to 6 times a year. Females are much smaller than males and can often be seen in a precopula, with the male straddling the female.
They are detritivores, meaning they feed on plant or animal debris. When they eat, they expel small fragments of plant or animal matter, which the benthic microfauna love.
Native gammarids can live in all types of habitat, but are very easily found under rocks or in aquatic plants. They are also threatened by an exotic species from Eastern Europe, the Danube gammarus. The latter is carnivorous and some of its prey are native gammarids.
Gammarids are also the prey of many animals such as planarians (flatworms), leeches, certain fish and birds.
Another exotic species that has recently colonised Lake Geneva is the Chelicorophium curvispinum. Originating in the Ponto-Caspian region, it has spread throughout Europe since the 20th century via canals, or by hanging its tubes from boat hulls.
Isopods are a large taxonomic group characterised by a dorso-ventrally flattened body. Most species are marine, but there is a large group of terrestrial species, including woodlice, numerous parasitic species and freshwater species such as the Aselles that can be seen in Lake Geneva.
Isopods also moult throughout their lives, but in two stages: first the posterior half, then the anterior half. It is therefore possible to observe animals where the back of the thorax is much wider than the front. This apparent anomaly is simply proof that the animal has not yet completed its moult.
During the breeding season, lamellar plates, or oostegites, develop on the ventral surface of certain segments of the female thorax. Together with the ventral wall of the body, these oostegites form an incubator pouch, or marsupium, in which the eggs develop.
Aselles generally lay their eggs twice a year, with 100 to 200 eggs per clutch.
Growing up to 1.5 cm, this species is a detritivore and preys on planarians, leeches, fish and birds.
Finally, the Mysidacea are an order of small crustaceans resembling shrimps. They are distinguished from shrimps by the presence, in females, of a marsupium under the thorax. This brood pouch is closed by oostegites. It is inside this pocket that the eggs are incubated. During reproduction, the male inserts his penis into this pouch and releases his spermatozoa, which stimulates the female to release the eggs. The embryos develop directly in the brood pouch, with the young hatching from the eggs as miniature adults.
The head of the Mysidacea contains a pair of pedunculated eyes and two pairs of antennae. The thorax is made up of eight segments, each bearing branched limbs, all covered by a protective carapace. The first two thoracic segments bear maxillipeds, which filter plankton and organic particles suspended in the water. The other six pairs of thoracic appendages are limbs that are used for swimming and for circulating water to the maxillipeds for feeding. Mycidaceans are therefore omnivorous filter feeders, feeding on algae, detritus and zooplankton.
These organisms are sensitive to water quality and are therefore sometimes used as bioindicators.
The Mysidaceae are represented in Lake Geneva by the recent appearance of the blood-red shrimp. This invasive species originates from the Ponto-Caspian basin. Its invasion is having a major impact on the composition of zooplankton.
♣ Species present in our book of determination sheets (sold in our shop)
The species can be recognised by its
whitish colour of the underside of the claws. Widespread throughout the Lake Geneva basin, it is the only species indigenous to the lake. Unfortunately, it has disappeared from the lake due to a number of pressures: overfishing, competing species and disease. However, it can still be found in certain rivers, and action is being taken to encourage it.
Measuring up to 18 cm, the Signal crayfish is the largest crayfish in Lake Geneva. 
It is easily recognised by the white spot on its large reddish claws. The signal crayfish is found in harbours and riprap. Opportunistic, its diet is mainly meat. It is particularly predatory on other, smaller crayfish species. Introduced from California in the 1980s, it is thought to have been introduced to replace native species affected by a disease of which it is a healthy carrier. ♣
Bearing numerous sharp spines, it is easily recognisable. Brown spots on the back can also be seen. A detritivore and piscivore, it is particularly fond of small fish such as minnows and sticklebacks.
The young also feed by filtering plankton. Like the Signal Crayfish, the American Crayfish was introduced to compensate for the decline in native populations. This introduction was the coup de grâce for local species. ♣
Two species of Gammares are found in the
Lake Geneva. Although they are very similar morphologically, the two species are distinguished more by their habitats. The Gammarus fossarumthe Pro Natura Animal of the Year for 2021, tends to live in rivers and cooler waters. In contrast, the Gammarus pulex is found in calm, temperate waters. These two species have been heavily impacted by the arrival of the Danube Gammarus. This has resulted in a gradual displacement of native Gammares towards deeper areas (>10 metres) not yet colonised by the invasive species.
Characterised by two dorsal spikes at the end of its body, it is also large (up to 3 cm for an adult male). Native to the Lower Danube, the Danube Gammarus has rapidly conquered European aquatic ecosystems. Its mass arrival in Lake Geneva dates back to 2001.
Two years later, significant observations were made on the seabed between 0 and 5 metres. Its progress was undoubtedly favoured by the presence of zebra mussels and quaggas, two invasive species from the same region as the Danube Gammarus. As these three species have co-evolved, there are synergies in their development.
Corophiidae are easily distinguished from other amphiphods by their highly developed antennae, which have been modified to form a kind of leg. Chelicorophium curvispinum is a species of Ponto-Caspian origin, and has been expanding towards Western Europe since the beginning of the 21st century. Its movements have been via canals. Navigation has undoubtedly accelerated the colonisation process. This species lives in tubes that it builds on hard substrates, particularly on the hulls of boats. It can therefore be transported from one body of water to another.
Aselles are easy to recognise because their bodies are flattened dorsoventrally and they have two pairs of antennae, one of which is very long. There are 8 plates on their body, 7 of which have thin legs. Aselles also moult throughout their lives and can reach a length of 1.5 cm. They are detritus feeders and are prey for planarians, leeches, fish and birds.
Asellus aquaticus is of Asian origin and competes with native species.
Mycidaceae resemble small shrimps with pedunculated eyes. Order
A newcomer to Lake Geneva, the Mysidas are represented by the blood-red shrimp, which takes its name from its red colour. This small Black Sea shrimp was first recorded in 2007. Since then, colonies of very large numbers have developed. For the moment, the environmental impact of this new arrival is not yet known, but it could unbalance zooplankton populations. ♣
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